This invention relates to a process to produce a coal-water mixture comprised of coal particles in an aqueous liquid medium. More particularly, the present invention relates to a process for producing a coal-water mixture from feedstock formed of coal particles which can be ground, freshly-mined coal or coal salvaged from silt ponds or other sources after processing to remove clay, shale, pyrite and other minerals wherein the feedstock is treated to impart dilatancy to the coal particles and two or more feed streams comprised of differently-sized, e.g., classified, coal particles in a liquid medium are mixed together with a dispersing agent to form a coal-water mixture having at least 65% by weight coal particles.
In my copending application Ser. Nos. 489,568 and 598,979, filed Apr. 28, 1983 and Apr. 16, 1984, respectively, there is disclosed a process for removing sulfur and ash from ultrafine coal using a feedstock which may be freshly-mined coal or coal salvaged from silt ponds or other sources. It is suitable, according to the present invention, to use the product from this process to form a coal-water mixture. One characteristic of the coal recovered from silt ponds is a substantial variation to the coal particle size distribution in a flow stream on a day-to-day basis and possibly on an hour-to-hour basis of operation of the process. A substantial variation to the particle size distribution of ultrafine sizes of freshly-mined coal can be expected when preparing feedstock for a process to form a coal-water mixture. The problem of variations to the particle size distribution of the feedstock exists in all currently-known methods for wet and dry grinding of coal.
In a paper entitled Rheology of High Solids Coal-Water Mixture by D. R. Dinger, J. E. Funk, Jr. and J. E. Funk, Sr., 4th International Symposium on Coal Slurry Combustion, May 10-12, 1984, there is described the "rheological properties" of a coal-water mixture having 98.5% coal particles at 50 mesh or less depending on the particle-packing efficiency which minimizes interstitial porosity. An equation for optimum particle-packing efficiency is derived and an algorithm developed calculating the porosity of real particle distributions. The calculated porosity was checked by pressure filtration and measurement of porosity. The specific surface area is also calculated by an algorithm. The data provides a family of particle size distributions which produce exceptional rheological properties provided that a surfactant addition is effective for dispersing the coal particles. It was found that monospheres, regardless of their size will usually pack to an average orthorhombic array of about 60% by volume. In order to shear, the structure must open or dilate to a cubic array where the porosity increases from 40% to about 48%. It was found that to prevent dilatancy, or interparticle collisions in shear, the system must be diluted so that the interparticle spacing is at least IPS-(2-.sqroot.3)D, where IPS is the interparticle spacing and D is the particle size.
The problem arises, however, as to the manner by which a coal-water mixture can be produced comprising at least, for example 65% by weight coal particles and preferably 70% and up to about 82% by weight coal particles on an hourly and day-to-day basis for reliable use. At about 65% by weight coal particles, a coal-water mixture requires the use of additional fuel such as a combustible gas when used in a power plant. However, the coal-water mixture can be economically utilized. It is, however, far more economical to provide a coal-water mixture with a coal-particle concentration of at least 70% by weight coal particles. Above 82% by weight coal particles, mechanical problems can be expected to impede delivery of the coal-water mixture by piping networks, pumps and valves.
Feedstock for a coal-water mixture is usually an aqueous coal slurry at about 20% to 40% by weight coal particles. The slurry must be dewatered to an extent sufficient to form a flowable coal-water mixture with at least 65% by weight coal and rheological properties, particularly viscosity that will not impede flow in pipelines at normal ambient temperatures, e.g., 0.degree. C. to 35.degree. C. It has been discovered that dilatancy of coal particles can be effectively utilized for dewatering a mass of coal particles derived from an aqueous coal slurry. It has also been discovered that dilatancy can be imparted to coal particles by increasing the ratio of surface area to mass whereby a dispersing agent in a subsequently-formed coal-water mixture functions in a surprising and far superior manner to enhance the flow characteristics of the mixture. The feedstock for the coal-water mixture can be made dilatant also by removing a clay constituent that is hydrophobic and prevents dilatancy.